The effects of symbiotic state and nutrient availability on the cnidom in the model sea anemone, Exaiptasia diaphana

Abstract

The symbiotic relationship between corals and zooxanthellae allows coral reefs to thrive. The symbiont translocates nutrients which it derives from photosynthesis to the coral, which then uses the nutrients for its growth, reproduction, metabolism, and the maintenance of structural integrity. The majority of the coral’s nutritional input is lost when the coral loses its symbionts in a bleaching event. However, the importance of the symbiotic state to the cnidom of corals remains unknown. The facultative symbiotic anemone Exaiptasia diaphana, common name Aiptasia, is widely used as a model organism to understand the symbiotic relationship between coral and their symbionts. We experimentally rendered Aiptasia aposymbiotic using a menthol treatment. We exposed symbiotic anemones and aposymbiotic anemones to differing feeding regimes to investigate the impact of nutritional input on the cnidom. Symbiotic state and feeding regime significantly affected the: ratio of cnida types in tentacles; the mean volume of cnidae; and the mean density of cnidae in Aiptasia. When Aiptasia becomes aposymbiotic, its tentacles possess more spirocysts and fewer penetrant-type nematocysts. Furthermore, cnidae in aposymbiotic anemones tended to be smaller than in symbiotic anemones. In addition, the ratio of cnida types and the mean volume of cnidae were significantly affected by nutrient availability in aposymbiotic anemones. When nutrients were exogenously supplied, the cnidom of aposymbiotic anemones became comparable to that of symbiotic anemones. Moreover, the mean size of cnidae significantly increased. Thus, the cnidom is a dynamic entity that is significantly influenced by the symbiotic state of the anemone and/or the availability of nutrients.